High aspect ratio thin film structures are conventionally formed by photolithography and metallization, typically by plating. The thin film structures may include thin film microelectronic circuitry, thin film magnetic transducer structures, and microelectronic mechanical micro actuators.
Typically, metallization areas of substrates are provided with seed layers to promote the plating or deposition of the metal on the substrate. Adhesion layers have been employed for providing good adhesion between the substrate and the seed layers. For example, see coassigned U.S. Pat. No. 5,126,016, which provides a chromium (Cr) adhesion layer for thin film microelectronic circuitry. IBM Technical Disclosure Bulletin, Vol. 38, No. 6, Page 625, June 1995, discloses a nickel iron (NiFe) adhesion layer for a copper (Cu) seed layer for plating a coil copper layer for magnetic recording transducers.
In addition to the seed layer, the developing of the photo resist material overlying the seed layer needs to be complete to provide good definition of the thin film structure at the seed layer, and to provide vertical walls for the high aspect ratio thin film structure. In high aspect ratio structures, the trench should go to the bottom and fully reach the seed layer, but tends to round off if not completely exposed. It is difficult for the imaging light to penetrate to the bottom of the trench in such high aspect ratio structures while maintaining the desired feature dimension. "Scumming" and "foot" are remnants of photo resist that are not removed in the developing of high aspect ratio features. They are caused by either inadequate exposure or an insufficient developing. The developing, however, must be stopped before adversely affecting the feature dimension. Thus, it is difficult to complete the exposure and developing of the high aspect ratio structure with good critical dimension definition. Barrier layers have been used in controlling the developing processes for subsequent metallization. For example, U.S. Pat. No. 5,591,480 provides a titanium barrier layer to control liquid etching and provide a metallization pattern having various thicknesses and different metals exposed. IBM Technical Disclosure Bulletin, Vol. 26, No. 7A, Page 3357, December 1983, discloses a titanium layer to act as an etch stop in a complicated image transfer method for making a high aspect ratio plated structure. Such barrier layers themselves must be removed from the seed layer before metallization of the seed layers may be conducted. A difficulty of removing such a barrier layer is the potential damage of the seed layer as the result of the removal.
The adhesion between the typical seed layer and typical photo resist material is not very strong, due to the nature of the seed layer, such as copper (Cu) or nickel iron (Ni--Fe), and the organic photoresist. The inadequate adhesion may cause lift off, or delamination, of the photo resist under stress.
The photolithography and metallization of high aspect ratio thin film structures cause high stress which may lead to adhesions failure and plating under the photo resist defining the metallization area. First, the developing must be complete to provide a high definition of the metallization area. However, the extra developing required to insure that the developing is complete, tends to etch into the sidewalls and to stress the adhesion between the photo resist layer and the seed layer. Other processes than extra developing may be used to complete the removal process, such as aggressive ashing, using oxygen (O.sub.2). Such ashing, however, may cause seed layer oxidation and may cause critical dimension loss as the O.sub.2 starts to etch laterally. Second, the metallization, typically by plating, causes stress, which may result in delamination of the photo resist from the seed layer. Any tendency toward separation between the photo resist and the seed layer resulting from the above stresses, may therefore result in underplating. Plating under the photo resist would result in thin film metal structures that are not well defined, destroying the thin film critical dimensions, possibly leading to changes in the characteristics of the circuits, inefficient thin film magnetic transducers, and poor performance actuators.
In addition, the excessive etch or the ashing to provide complete removal of the photo resist material or of a barrier layer may adversely affect the effectiveness of the seed layer, e.g., if oxidized, and not insure an effective plating process.